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A total of 30 synthetic samples of the Ca 2 Fe 2-x Al x O 5 , 0.00 ≤ x ≤ 1.34 solid solution series have been investigated by single crystal X-ray diffraction at 25 °C. Pure Ca 2 Fe 2 O 5  and samples up to x = 0.56 have space group Pnma, Z = 4, whereas samples with x &gt; 0.56 show I2mb symmetry, Z = 4. The substitution of Fe 3+  by the smaller Al 3+  cation decreases unit-cell parameters and average octahedral and tetrahedral bond lengths and induces distinct changes in the O-atom coordination of the interstitial Ca atom. Discontinuities in the structural parameters vs. the Al 3+  tot  content and changes in slope of these quantities are associated with the phase transition. The essential difference between the two modifications is the cation-O atom-cation angle within the planes of corner sharing octahedra, which is close to 180° in I2mb, but ≈184° in the Pnma phase, and the existence of two different orientations of the tetrahedral chains in Pnma as opposed to one in I2mb. At low overall Al 3+  concentrations Al 3+  preferentially enters the tetrahedral site until ≈2/3 of it is filled. Additional Al 3+  cations, substituted for Fe 3+ , are equally distributed over octahedral and tetrahedral sites. At high temperature pure Ca 2 Fe 2 O 5  transforms to a body-centered structure at 724(4) °C. Substituting Al 3+  for Fe 3+  linearly decreases the transition temperature by 15 °C per 0.1 Al 3+  down to 623(5) °C for x = 0.65.

american mineralogist

Structural variations in the brownmillerite series Ca 2 (Fe 2–x Al x )O 5 : Single-crystal X-ray diffraction at 25 °C and high-temperature X-ray powder diffraction (25 °C ≤ T ≤ 1000 °C)